New in Symfony 3.3: Service autoconfiguration

_defaults: defines the default value for the public, tags and
autowire options of the services defined in a given file;

_instanceof: defines the default configuration of services depending on
their classes (e.g. add the twig.extension tag to any service that
implements Twig_ExtensionInterface).

The evolution of this simplification is the new autoconfigure option, which
is like an automated version of _instanceof. If enabled, this option adds
some default configuration depending on the class implemented by the service.

Let's suppose that you want to add tags automatically to your security voters:

Now ask yourself: if you are registering a service with a class that implements
VoterInterface, when would you ever not want that to be tagged with
security.voter? In other words, a service implementing VoterInterface
can only be a security voter, unless you are doing some seriously weird things.

This works because each enabled bundle has the opportunity to add one or more
automated _instanceof definitions. Of course we've already enabled this for
all the common Symfony services: commands, form types, event subscribers, etc.

Whenever we introduce a new feature to automatize the configuration of services,
some developers quickly discredit it for being "magic". For us, "magic" means
that something happened without you explicitly asking for it. Magic is bad
because it leads to WTF moments and hard-to-debug issues.

However, this feature won't work unless you explicitly include the
autoconfigure option in your configuration file. Besides, it only applies
to the services defined in the same file where you include autoconfigure,
so there will be no side-effects. In short, this is not magic, just automation.

"This works because each enabled bundle has the opportunity to add one or more automated _instanceof definitions. Of course we've already enabled this for all the common Symfony services: commands, form types, event subscribers, etc."

@Niels Keurentjes THIS IS MAGIC. It's already double magic actually with adding tags and they're auto-adding just to refer and interface.

This has no added value for end user at the moment, since instead of "tags: sth" I just write global per-file "autoconfigure". I even had to go to PR to find out what this word stands for this, since "autoconfigure" tell me the same none information as "automanager".